ION-ICE ASTROCHEMISTRY: BARRIERLESS LOW-ENERGY DEPOSITION PATHWAYS TO HCOOH, CH3OH, AND CO2ON ICY GRAIN MANTLES FROM PRECURSOR CATIONS

Abstract

A new family of very favorable reaction pathways is explored involving the deposition of ions on icy grain mantles with very low energies. Quantum chemical cluster calculations at the MP2/6-31+G** level in 4H2O clusters and at the B3LYP/6-31+G** level in 17H2O clusters indicate that HCO+ and CH3+ are able to react spontaneously with one of the water molecules in the cluster to form protonated formic acid (HCOOH2+) and protonated methanol (CH3OH2+), respectively. It is furthermore found that these initial adducts spontaneously transfer their excess protons to the cluster to form neutral formic acid and methanol, plus solvated hydronium, H3O+. In the final case, if a CO molecule is bound to the surface of the cluster, OH+ may react with it to form protonated carbon dioxide (HCO2+), which then loses its proton to yield CO2 and H3O+. In the present model, all of these processes were found to occur with no barriers. Discussion includes the analogous gas-phase processes, which have been considered in previous studies, as well as the competitive abstraction pathway for HCO+ + H2O.